Diffusion pump



Jan. 9 1968 A. A. LANDFORS DIFFUSION PUMP 2 Sheets-Sheet l I Filed July 13, 1966 coouNs WATER m. A 6 2 I I R O T E C OcOcCOoO q H B E N mm 3 U N m I. T wm m C O D FIG.2

Jan. 9, 1968 A. A. LANDFORS 3,362,622

DIFFUS ION PUMP Filed July 13, 1966 2 Sheets-Sheet 2 TOP NOZZLE '5 ISI I34 I .i'; =J z 1 United States Patent 3,362,622 DIFFUSION PUMP Arthur A. Landfors, Sharon, Mass, assignor to National Research Corporation, Newton, Mass, a corporation of Massachusetts Filed July 13, 1966, Ser. No. 564,771 Claims. (Cl. 230-101) ABSTRACT 0F THE DISCLOSURE The present invention relates to oil diffusion pumps and has particular application to oil diffusion pumps used in repetitive rapid cycle operations such as cathode ray tube aluminizing and utilizing chemically stable oils (such as silicone oils) which may be heated at atmospheric pressure. This application is a continuation-in-part of S.N. 522,523.

In operations such as aluminizing, a series of pumping systems is provided. Each pumping system has a diffusion pump with an inlet and a discharge foreline connected to a mechanical backing pump. A tube to be aluminized is connected to the diffusion pump inlet. The mechanical pump rough pumps the tube through the diffusion pump and open foreline valve. Meanwhile the oil in the diffusion pump is heated. When the pressure in the diffusion pump is sufficiently low, the oil boils and is formed into jets which provide fine pumping to harden the vacuum for the aluminizing operation in the tube. After a predetermined time interval, the pumping system is air released, the aluminized tube is unloaded, and a new tube is loaded.

In order to get maximum cycle speed, it is desirable that the diffusion pump shall have the capability to break its jets very quickly, prior to air release, and to reform the jets very quickly in the pumpdown phase of the next subsequent cycle.

Objects of the invention The principal object of this invention is to provide an improved diffusion pump which meets the foregoing criteria with structural changes in the pump which involve very little extra ost compared to conventional pumps, yet offer reliable performance in meeting the foregoing criteria to provide substantial improvement in the time and quality of the cyclic processes and systems incorporating such improved pumps.

In order to meet this principal object I invented an improved pump with a spiral coil-baffle arrangement which demonstrated excellent suppression of oil loss and allowed rapid cycle operation. I then discovered that during the pumping cycles, per se, pumps incorporating the invention exhibited surprisingly improved throughput.

It may therefore be stated as a further object of the invention the provision of an improved diffusion pump exhibiting a high throughput rate.

Description of the invention The invention is now described in detail with reference to the accompanying drawings wherein:

3,362,622 Patented Jan. 9, 1968 FIG. 1 is a diagrammatic view of a pumping system incorporating the improved diffusion pump of the invention, which is illustrated in simplified form;

FIG. 2 is a sectional view showing the diffusion pump as actually constructed and used in the first working example given herein;

FIG. 3 is a partly sectional view of a second embodiment of the invention; and

FIG. 4 is a partly sectional view of a third embodiment of the invention; and

FIG. 5 is a partly sectional view of a fourth embodiment of the invention.

Context i Referring now to FIG. 1-, the elements of each pumping system are the tube to be evacuated, a connector, a diffusion pump 10, a foreline 20, with a foreline valve 22 and a mechanical backing pump 39. The diffusion pump (the lower portion of which is shown in the drawing in simplified form) comprises a pump body 11 with cooling coils 12. A pool of silicone pump oil 13 is contained in a reservoir at the bottom of the pump body and held at high temperature by a heater 14. A vajor jet nozzle assembly 15 is contained in the body. Boiling oil vapors from the reservoir 15 rise up within the assembly 15 and are expanded through the nozzles 15A, 15B, 15C, etc. into the annular pumping space around assembly 15 to pump air. The oil vapors are collected on the cooled wall 11 as condensate and they drip down the wall to return to the reservoir. Holes 13A in the lower portion of the vajor jet assembly 15 provide a return path to the interior of the vapor jet assembly. A foreline trap (not shown) may be provided the foreline 20 to limit oil carryover.

The pump is charged with a silicone oil such as DC704. In a four-inch diffusion pump, the charge of oil would be 300 cc., preferably to provide an oil level between and 4 inch. The advantage of such oils is that they can be held at operating temperature while the. system is raised to atmospheric pressure. This allows rapid cycling, but unfortunately increases the dangers of oil loss. Frequent addition of oil to maintain an adequate level is undesirable as a matter of oil cost and is inconsistent with the needs of an automatic, low maintenance, aluminizing line.

Prior art There are several approaches to limiting oil loss. First, the above-noted foreline trap limits oil carryover from the diffusion pump to the mechanical pump. But the reduction alone is insufficient when it is desired to run several hundred cycles without maintenance. A second approach involves cutting off the pump heater to cool the pump oil so that it does not escape and this approach may be augmented by an internal quench coil in or adjacent the oil pool, as described in the patent to Gerow, et a1., 2,933,233. But the time required to heat and cool is still a significant obstacle to rapid cycle operation. Also, heater lifetime is limited if the heater is shut down at the end of each cycle. Third, it is possible to provide an inlet valve for the diffusion pump. But expense and added maintenance and reliability problems are disadvantages of this approach.

In the past I have developed (with Steinherz) a fourth approach to reducing oil loss through modification of the pump construction. A baffle is placed above the oil pool. Just before an air release (about 4 seconds before) water is admitted to the baflie to cool it. The jets of the pump are quickly broken by typical operating forepressures (50 microns and up). The oil pool temperature stays at its operating level while the baffle condenses vapors above the pool. When the pump is air released, boiling stops. Once baffle cooling is stopped, the diffusion pump can take hold again in short times (in the next subsequent pumping cycle against high forepressures, e.g., reforming General description The present invention involves a further improvement over the fourth approach described above affording a more complete suppression of oil loss. According to the principal feature of the present invention, an internal cooling coil of cylindrical spiral configuration is placed above the pump oil reservoir. An annular disc is placed around the coil about halfway up its length and a solid disc is placed above the center of the coil. This arrangement causes a majority of the vapors produced in the oil pump reservoir to make double transit of the coil (going into and out of the coil). When cooling water is admitted to the coil, the vapors in transit through the coil are condensed on the coil turns and dripped back into the oil pool. After air release, air coming into the pump enters the vapor jet assembly through the nozzle and, as the air pressure in the assembly rises, boiling stops. At the time of the air release there is no significant oil vapor in transit within the pump. When the present invention is used, essentially the only oil vapor available for forming part of the mist is the small quantity of oil vapor within the cooling coil. However, DC. 704 has a vapor pressure of about 1 millimeter of mercury at 440 F., the normal boiling temperature. Even though the atmospheric pressure on the surface of the oil has suppressed boiling, there will still be some evaporation which becomes mist. The oil loss from this source can be disposed of by apparatus of the type described in U.S. Patent 2,902,206 to Power.

Specific description Referring to FIG. 1, the internal cooling coil is indicated at 16. Vapor flow through the coil, as defined by the baflles 18 and 19, is indicated by the arrows X. Baffle 18 is a solid disc soldered to the top of the coil and bafiie 19 is an annular disc (a washer) soldered to the wall of the jet assembly 15. While the baflie 18 inherently turns cold during cooling of the coil 16, the temperature of this bafile is not critical to working of the invention. The cool. ing coil itself has the form of a cylindrical spiral which allows easy access to the bottom of the pump body for cleaning (after the jet assembly 15 is lifted out through the pump inlet). It is preferred that the space between turns will be equal to about one to two pipe diameters. The total escape area between turns (above bafiie 19) should be greater than the total escape area from nozzles 15A, 15B, 15C, etc. to prevent choking. The coil construction is simple and inexpensive to manufacture and it has the further advantage that is free to expand and contract in response to thermal variations so that it is not stressed. The ends of the coil are brought out of the pump through the side wall via two seals (one of which is indicated at 17) located at the bottom of the pump below the oil level. The use of feedthroughs in the side wall of the pump avoids interference with the heater assembly.

The foreline valve 22 can be bypassed through a valve 24 (typically, inch opening compared to a one-inch opening for the foreline valve). An air release valve for the system is indicated at 26. A timer 28 controls operation of the above valves 22, 24, 26 and also controls operation of the cooling coil through a valve 32.

Referring now to FIG. 2, the lower end of the diffusion pump is shown in more detail. The boilerplate 6 has a finned construction, known per se. The operating oil levels are measured from the tops of fin. The internal cooling coil has a portion 7 which extends below the top of oil pool 13 to complete the spiral over its full height for simplicity of construction and more stable support. The portion of the coil 16 passing through the oil pool 13 (feed and discharge runs and the spiral turns 7) have little efiect on oil temperature because of their limited surface area exposed to the oil and because the pump heater 14 operates continuously. The vapor jet assembly 15 and the pump body have annular dimples at 8 and 9, respectively, to limit backstreaming from the portion of the oilpool outside assembly 15.

Example 1 A pump was constructed as shown in FIGS. 1-2. The pump was subjected repeatedly to the following 7-minute cycle:

Time zero: Admit water to coil 16; close foreline valve 22;

air release valve 26 stays closed,

Zero plus 15 seconds: Open air release valve,

Zero plus 45 seconds: Cut off water to coil 16,

Zero plus seconds: Close air release valve, open foreline valve,

Pumpdown from zero plus 120 to zero plus 420 seconds.

This pumping cycle was run 550 times, consecutively, and resulted in a total oil loss of 25 cubic centimeters.

FIG. 3

Referring now to FIG. 3, another embodiment of my invention is shown. This incorporates a conventional fractionating tube 151. In this embodiment, the disc 18 takes the form of a flange secured to the tube 151. Sufficient space is left between the inner diameter of the spiral coil and the outer wall of the fractionating tube to allow passage of oil vapors without choking. The coil going up the fractionating tube avoids regulation by coil 16, but oil loss and backstreaming from this source are so small that they do not have a significantly adverse effect.

FIG. 4

Referring now to FIG. 4, an embodiment of my inven tion is shown for use in a pump having a conventional tiedown rod 152. The baflle 18 is held to the tie down rod by a pair of nuts 181 threaded on the rod. The construction is otherwise the same as FIG. 2.

The pumps of FIGS. 1-4 also exhibit improved throughput due to the presence of the coil 16 with the double transit path formed by baffles 18, 19. The surprising result was noted in the operation of pumps built as shown in FIG. 2. This observation suggests a still further embodiment of the invention which has been successfully practiced and is described below in connection with FIG. 5.

FIG. 5

Referring now to FIG. 5 another embodiment of the improved pump is shown. In this embodiment the pump 11 is a commercially available pump marketed under the model designation VHS. The pump comprises the usual vapor jet assembly 15 with a nozzle 15A, other jet nozzles (not shown), and a fractionating tube 151, a boilerplate 6 with a pool of oil 13 at the bottom of the pump and an oil seal 13A formed by an annular column of oil between the pump wall and vapor jet assembly.

A metal cylinder 161 is placed centrally in the pump. The cylinder extend upwardly from the oil pool 13. Holes 162 are drilled in the cylinder. The cylinder is capped by a baffle 18 and surrounded by a baffie 19 as in the previous embodiments. However, the baffle 19 extends only to the wall of an outer cylinder 191 and not to the vapor jet assembly 15. Thus, a secondary annular oil column 133 is formed to limit heat transfer to the oil seal 13A.

This construction of cylinder 161 and the baffles 18, 19 provides a double transit for a substantial portion of the oil as shown by the arrows.

Example 2 A VHS-4 pump having a nominal diameter of 4 inches was operated without a baffle array as shown in FIG. 5. The maximum throughput obtainable with no bafiie or with several other types of bafi le arrays was less than 2000 micron-liters per second.

The pump was then operated with the annular screen 5 and battle array shown in FIG. 5. The cylinder 161 had 24 drill holes of inch diameter above baflle 19 and 48 holes of /8 inch diameter below bafile 19. The pump exhibited a throughput of 2800 at 3 microns inlet pressure and 2480 at 2 microns inlet pressure.

The pump of FIG. 5 can also be equipped with cooling means, as in the other embodiments, for cooling cylinder 161 at the end of each cycle to limit oil loss.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that modifications may be made by those skilled in the art Without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as well as the embodiments described herein.

What is claimed is:

1. An improved oil diffusion pump comprising, in combination:

(a) a conventional pump body with a boiler;

(b) a conventional vapor jet assembly;

(c) an internal annular screen enclosing a central volume in the boiler region of the pump body;

(d) bafile means surrounding the screen to limit gas flow to a path making a double transit of said screen so that boiling oil vapors from said boiler enter the said enclosed volume at the lower portion thereof and exit from the upper portion thereof, whereby improved throughput is obtained.

2. The pump of claim 1 wherein said screen is a perforated cylindrical tube.

3. The pump of claim 2 wherein said screen is provided with selectively operable cooling means for cooling the screen at the end of a pumping cycle.

4. The pump of claim 1 wherein said bafiie means comprise a central baffle at the top of said annular screen and an annular baflle surrounding the screen and axially located at an axially central portion of the screen.

5. The pump of claim 1 wherein the vapor jet assembly, pump body and boiler are constructed and arranged to provide an annular oil seal in the boiler region and wherein said screen and baflie means are surrounded by an annular wall constructed and arranged to form an annular oil column, in cooperation with said vapor jet assembly, between said oil seal and baflle means.

References Cited UNITED STATES PATENTS 2,585,139 2/1952 Lawrance et al 230-101 2,840,297 6/1958 Hickman 230101 3,227,361 1/1966 Erhart et al. 230-101 3,302,864 2/1967 Nicolas 230101 DONLEY'I. STOCKING, Primary Examiner.

W. J. KRAUSS, Assistant Examiner. 

